Method and Apparatus for Measuring Interference in a Telecommunications System

ABSTRACT

A receiving unit or a transmitting unit of a wireless network, for facilitating noise and interference measurements on an air interface. The receiving unit includes: a network interface which obtains information relating to a time-varying layout of a silent resource element (RE) grid; an electronic memory having code in communication with the network interface which stores the information; and a processing unit in communication with the electronic memory that takes a presence of a silent RE grid into account in the reception of a control or a data channel. 
     A method for a receiving/transmitting unit of a wireless network, for facilitating noise and interference measurements on an air interface is described. Information relating to a time-varying layout of a silent resource element (RE) grid is obtained and a presence of a silent RE grid is taken into account in the reception of a control or a data channel.

CLAIMING BENEFIT OF PRIOR FILED U.S. APPLICATION

This application is a continuation of U.S. patent application Ser. No.13/744,644 which is a Continuation of U.S. patent application Ser. No.12/365,703, filed Feb. 4, 2009, which claims the benefit of U.S.Provisional Application No. 61/026,230, filed Feb. 5, 2008, and thecontents of all of the preceding are hereby incorporated by referenceherein.

TECHNICAL FIELD

The present invention relates to a method and arrangement in atelecommunications system. (As used herein, references to the “presentinvention” or “invention” relate to exemplary embodiments and notnecessarily to every embodiment encompassed by the appended claims.)More specifically, the present invention relates to facilitatingInter-cell interference measurement.

BACKGROUND

This section is intended to introduce the reader to various aspects ofthe art that may be related to various aspects of the present invention.The following discussion is intended to provide information tofacilitate a better understanding of the present invention. Accordingly,it should be understood that statements in the following discussion areto be read in this light, and not as admissions of prior art.

The invention is described with some references to LTE, but it should benoted that it conceivably can be applied to other systems as well,including e.g. WCDMA. It should also be noted that the invention isequally applicable both in the downlink as well as the uplink of awireless system.

Signal and channel quality estimation is a fundamental part of a modernwireless system. The noise and interference is used not only in thedemodulator, but is also an important quantity when estimating, forexample, the channel quality indicator (CQI). The following is notedwith LTE in mind, but the principles are valid for most wirelesssystems.

The interference can be estimated from the common reference symbols (RS)that are present in the time-frequency grid of an OFDM based system(e.g., LTE). An element in the time-frequency grid is referred to as aresource element (RE). The received signal in a RE can be written asr=Hs+n, where H is the channel response and s represents the transmittedsymbols. The term, n, is the (unknown) noise and interference. The noiseand interference show different characteristics in resource elements(RE) that hold RS, data, and control signaling; we therefore denote thereceived noise/interference for the different signaling types by I_RS,I_d, and I_control, respectively.

It is noted that the interference term in a RE containing RS, I_RS, at aspecific user equipment (UE) can be estimated by the UE since s areknown symbols and H is given by the channel estimator. It is furthernoted that the interference on REs with data (that is scheduled for theUE in question), I_d, also can be measured as soon as the (data)symbols, s, are detected (at this moment they can be regarded as knownsymbols). Similarly, to estimate I_control, the control symbols mustfirst be detected.

BRIEF SUMMARY OF THE INVENTION

The statistical characteristics of the (inter-cell) interference may besignificantly different, depending on whether the interferenceoriginates from

1. reference symbols from neighbouring cells

2. data signalling from neighbouring cells

3. control signalling from neighbouring cells

Each of these three categories can have different transmission power andspatial characteristics.

For accurate data channel CQI computation the UE must have goodstatistics of the interference that hits the data channel (which is amixture of the three categories mentioned above), I_d. Ultimately, thisinterference statistics should be measured on the data channel itself;however, this measurement is limited to resource entities (i.e.,time-frequency slots) that contain data scheduled to the particularuser. The limited number of interference samples can significantlypenalize the accuracy of the statistics estimate. Moreover, inmultiuser-MIMO (i.e., spatial division multiple access) systems, severalusers may be assigned the same data REs, which in effect prohibits theUE to separate the inter-cell interference from the intra-cellinterference, if the measurement is performed on data REs.

Alternatively, the interference measurement can be performed on REscontaining a RS. However, the statistics of the interference that hitsthe RS, may have significantly different statistics than theinterference on the data channel (or the control channel). There is alimited set of RS, and in particular for MIMO, where the positionholding a RS on one antenna is empty for a neighboring antenna,therefore the interference hitting a RS will to a larger extent comefrom RSs of the neighboring cells. In particular in lightly loadedsystems, I_RS, may be significantly different (typically substantiallylarger) than I_d, because possibly data is not allocated to all resourceblocks (RB) in the neighboring cells. The statistics of the measuredinterference term may therefore deviate significantly from theinterference that hits the data channel. The RS grid for a RB in case of1, 2 and 4 Tx antennas is illustrated in FIG. 1. Between cells, the RSis shifted in frequency domain.

For two Tx antennas only three frequency shifts for common RS exists.This will lead to that not all data interference can be measured.Furthermore, the first three OFDM symbols might see control channelinterference instead of data interference. Since control signaling maybe differently power controlled than the data, the interference estimateobtained on these RS may not reflect the interference present when datais transmitted. If common RS in the later part of sub-frame is removed(for example because dedicated RS are inserted instead) it might benecessary to measure interference on data REs.

According to one aspect of the present invention a cell specifictime-varying grid of data REs (referred to as the silent RE grid in thefollowing) is introduced, for which no data is allocated; that is, thetransmitter will be silent on this grid. The grid is known to both thereceiving and the transmitting unit.

According to a further aspect of one embodiment of the present inventionthe receiving unit are able to measure the statistics of the signal thatis received on the silent RE grid; this signal, originates completelyfrom inter-cell signals that interfere with the data channel.

The measured inter-cell interference can, for example, be used as abasis for channel quality (CQI) reporting and/or for interferencesuppression purposes in the receiving unit.

One advantage according to an aspect of an embodiment of the presentinvention is that a statistics measurement, performed on the silent REgrid, is computationally trivial, because no data must be decoded toobtain the residual noise and interference.

According to a further aspect of another embodiment of the presentinvention, the silent RE grid, could be randomized in consecutive (infrequency and time) resource blocks, to minimize the effects ofpotentially overlapping grids of neighbouring cells.

According to a further aspect of another embodiment of the presentinvention, the layout of the cell specific silent RE grid can be derivedwith little, or no, signalling overhead; it could for example be derivedfrom the cell ID and other information that is known to both thereceiving and the transmitting unit.

According to a further aspect of another embodiment of the presentinvention, a similar approach can be used to estimate the interferencethat hit the control channel, I_control, by introducing another silentRE grid on the control channel (e.g., potentially up to the first threeODFM symbols in an LTE subframe).

According to one aspect of the present invention a method in a method ina receiving unit of a wireless network, for facilitating noise andinterference measurements on an air interface is provided. The methodcomprises the steps of obtaining information relating to a layout of asilent RE grid, and taking the presence of a silent RE grid into accountin the reception of a control or a data channel.

According to another embodiment of the present invention, the step ofobtaining information comprises using a pseudo-random sequence togenerate said layout of said silent RE grid, and synchronising saidpseudo-random sequency in each frame by deriving a seed for saidpseudo-random sequency from a Cell ID and a Frame Index.

According to another embodiment of the present invention the step ofobtaining information comprises receiving said layout of said silent REgrid through signalling.

According to another embodiment of the present invention the methodincludes the step of collecting statistics relating to the inter-cellinterference based on measurements on said silent RE grid, and usingsaid statistics to estimate any or all of: a channel quality indicator,the inter-cell interference power that hits the channel, the co-variancematrix of the inter-cell interference that hits the channel, the fullprobability distribution of the inter-cell interference.

Further characteristics of the invention and advantages thereof will beevident from the following detailed description of embodiments of theinvention.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will become more fully understood from thedetailed description of embodiments of the present invention givenherein below and the accompanying figures, which are given by way ofillustration only, and thus are not limitative of the present invention.

FIG. 1A is an RS grid for an RB in case of 1 antenna;

FIG. 1B is an RS grid for an RB in case of 2 Tx antennas;

FIG. 1C-1 is a first RS grid for an RB in case of 4 Tx antennas;

FIG. 1C-2 is a second RS grid for an RB in case of 4 Tx antennas;

FIG. 2 is a block diagram of a receiver unit of the present invention.

FIG. 3 is a block diagram of a transmitting unit of the presentinvention.

DETAILED DESCRIPTION

Referring now to the drawings wherein like reference numerals refer tosimilar or identical parts throughout the several views, and morespecifically to FIG. 2 thereof, there is shown a receiving unit 100, ora transmitting unit 110, as shown in FIG. 3, of a wireless network, forfacilitating noise and interference measurements on an air interface.The receiving unit 100 comprises a network interface 12 which obtainsinformation relating to a time-varying layout of a silent resourceelement (RE) grid. The receiving unit 100 comprises an electronic memory16 having code in communication with the network interface 12 whichstores the information. The receiving unit 100 comprises a processingunit 14 in communication with the electronic memory 16 that takes apresence of a silent RE grid into account in the reception of a controlor a data channel. In the transmitting unit 110, the processing unit 14takes the presence of a silent RE grid into account in the transmissionof a control or a data channel

The processing unit 14 can use a pseudo-random sequence to generate thelayout of the silent RE grid, and synchronizing the pseudo-randomsequence by deriving a seed for the pseudo-random sequence fromparameters. The pseudo-random sequence can be synchronized in each frameby the processing unit 14 based on the parameters that includes a cellID and a frame index. The layout of the silent RE grid can be receivedthrough signaling by the network interface 12.

The electronic memory 16 can collect statistics relating to inter-cellinterference based on measurements on the silent RE grid. The processingunit 14 can use the statistics with the code to estimate any or all of:a channel quality indicator, inter-cell power that hits a channel, aco-variance matrix of the inter-cell interference that hits the channel,a full probability distribution of the Inter-cell interference; and sendthe signal through the network interface 12. The processing unit 14 canrandomize the silent RE grid in consecutive, in frequency and time,resource blocks (RBs). The processing unit 14 can derive the silent REgrid with essentially no signaling overhead.

The processing unit 14 can of introduce another silent RE grid on acontrol channel to estimate the interference that hits the controlchannel using the code in the electronic memory 16. The processing unit14 can coordinate each silent grid of neighboring cells in the wirelesssystem such that there is no significant overlap. For each subframe, theprocessing unit 14 can group resource blocks into resource windows whereeach resource window is a group of contiguous, in frequency, resourceblocks; and the number of resource blocks in each resource window may beconfigured semi-statically. Each resource window has a same number ofsilent REs, and are configured semi-statically.

The present invention pertains to a method in a receiving unit 100 or atransmitting unit 110 of a wireless network, for facilitating noise andinterference measurements on an air interface. The method comprises thesteps of obtaining information relating to a time-varying layout of asilent resource element (RE) grid. There is the step of taking apresence of a silent RE grid into account in the reception of a controlor a data channel.

The step of obtaining information can include the steps of using apseudo-random sequence to generate said layout of said silent RE grid,and synchronizing the pseudo-random sequence by deriving a seed for thepseudo-random sequence from parameters known to both a receiving unit100 and a transmitting unit 110. The pseudo-random sequence can besynchronized in each frame based on the parameters that includes a cellID and a frame index. The layout of the silent RE grid can be receivedthrough signaling.

There can be the steps of collecting statistics relating to inter-cellinterference based on measurements on the silent RE grid in anelectronic memory 16 having code; using the statistics with the code toestimate any or all of: a channel quality indicator, inter-cell powerthat hits a channel, a co-variance matrix of the inter-cell interferencethat hits the channel, a full probability distribution of the Inter-cellinterference; and sending the signal. There can be the step ofrandomizing the silent RE grid in consecutive, in frequency and time,resource blocks (RBs). There can be the step of deriving the silent REgrid with essentially no signaling overhead.

There can be the step of introducing another silent RE grid on a controlchannel to estimate the interference that hits the control channel usingcode in an electronic memory 16. There can be the step of coordinatingeach silent grid of neighboring cells in said wireless system such thatthere is no significant overlap. For each subframe, there can be thestep of grouping resource blocks into resource windows where eachresource window is a group of contiguous, in frequency, resource blocks;and the number of resource blocks in each resource window may beconfigured semi-statically. Each resource window has a same number ofsilent REs, and are configured semi-statically.

In more detail, the invention contains the following general steps, andexample realizations. In the following we focus on the data channel, butthe same approach, with straightforward modifications, can be applied toestimate the interference that hit the control channel.

Designing the Silent Grids

Several aspects should be kept in mind when designing the silent grid.

The interference on the silent grid should reflect the interferencestatistics of the data channel as a whole. Therefore distribution ofsilent REs over the data channel should preferably be as uniform aspossible.

According to one embodiment the grid could be pseudo-random in the sensethat it should change over time and/or frequency to avoid significantlong-term overlap with silent grids of neighbouring cells.

It should be noted that the invention is not limited to pseudo-randomgrids; for example, an alternative method is to coordinate the silentgrid of neighbouring cells such that no significant overlap occur andthen signal the selected silent RE grid to the UEs dynamically orsemi-statically. Such an approach could however possibly require carefulplanning.

According to further embodiment the UEs should be able to readily derivewhich silent grid is being used, with little or preferably no controlsignalling overhead.

Alternatively it is conceivable to use explicit signalling of the silentgrid layout.

According to a further embodiment clustering of the silent REs in one ora few RBs should be avoided for improved sample statistics.

A simple scheme, according to one embodiment, that achieves some or allof the criteria mentioned above is as follows:

For each subframe, the RBs are grouped into resource windows (RW);

Each RW is a group of contiguous (in frequency) RBs;

The number of RBs in each RW is configured semi-statically;

As special cases, there could be only one RW that represent the entiresystem bandwidth, or there could be a RW for each RB.

For each RW, all REs that are not control or RS signalling areenumerated;

In each RW, a specified number of silent REs are selected from the setof enumerated REs using a pseudo-random number generator that generatesnumbers uniformly in the range of the enumerated data REs.Alternatively, selection of the silent RE are coordinated. The groupinginto RWs, will ensure that clustering of silent REs is sufficientlylimited. The number of selected silent REs could be fixed, derived fromthe number of RBs in each RW, or possible semi-statically configured.

The scheme can also readily be generalized to let a RW constitute only apart of a RB in the time domain; for example, the OFDM symbols in the RBcan be grouped so that symbols including RS symbols, and data symbols,are mapped to RW_RS and OFDM symbols carrying only data are mapped toRW_d. It is also conceivable to use a different number of silent REs inRW_(—) RS and RW_d. Such a separation of the OFDM symbols can be usefulsince the inter-cell interference that hit RW_RS and RW_d will havedifferent statistics.

According to a further embodiment the power that is saved by remainingsilent on some REs can be redistributed to boost the power on other REsthat carry data or reference symbols. Such power reallocation combinedby the preceding method of assigning different OFDM symbols to differentRW categories, can be a useful approach to reallocated the power withinOFDM symbols, for example in order to support different powers on the REcarrying RS and the REs carrying data in the same OFDM symbol.

Determining and Configuring the Silent RE Grid

As indicated in the preceding, it is beneficial to let the silent REgrid be generated pseudo-randomly to transparently avoid consistentgrid-overlap with neighbouring cells. It is therefore necessary tosynchronize the random generators used at the NodeB with those in theUEs.

According to a further embodiment of the present invention a simplescheme for synchronizing the pseudo-random generators is to reinitializethe pseudo-random generators in each frame using a seed that is derivedfrom

1. Cell ID, to provide uniqueness for each cell.

2. Frame index, to provide hopping over time.

Both the cell ID and the frame index are available at the UEs. Note thatany re-initialization interval of the number generator can be used, aslong as there is a suitable index, available at the UE, that can be usedto progress the seed over time; this includes, but is not limited to, asubframe, or groups of frames.

If more flexibility is desired, such as configurable silent RE griddensities, some grid parameters could be set semi-statically usinghigher layer control signalling.

Measuring the Interference

The silent RE grid is used by the UEs to collect statistics of theinter-cell interference that hit the data channel. If the silent grid isuniformly distributed over the data channel, as suggested in thepreceding, the interference samples collected in the silent grid will,over time, be the same as the interference that hit the data channel asa whole. These samples can thus be utilized by the UE to estimate:

-   -   The inter-cell interference power that hits the data channel.    -   The co-variance matrix of the inter-cell interference that hits        the data channel. This is particularly useful if the UE has        multiple antennas    -   The full probability distribution of the inter-cell        interference.    -   The measured statistics can be average over time and frequency;

similar to the approach proposed in R1-074855, CQI MeasurementMethodology, Ericsson, 3GPP RAN1#51, Korea.

According to one embodiment the interference measurement on the silentRE grid can, for example, be used as a basis for the CQI reporting if itis combined with measurements of the received power of the data signals.Such a signal power estimate can, for instance, be obtained frommeasurements on the RS, taking potential signalling-power normalizationseffects into account.

Exemplary improvements of different aspects and embodiments of thepresent invention is Improved capacity, coverage and quality, enabledthrough more accurate link adaptation, power control and scheduling, inturn enabled by more accurate interference estimation.

Moreover, the invention allows for inter-cell interference measurementsdirectly on the data channel with small computational-complexity. Thescheme also transparently manages inter-cell interference measurementsin multiuser-MIMO allocation scenarios.

Although the invention has been described in detail in the foregoingembodiments for the purpose of illustration, it is to be understood thatsuch detail is solely for that purpose and that variations can be madetherein by those skilled in the art without departing from the spiritand scope of the invention except as it may be described by thefollowing claims.

1. A method in a receiving unit of a wireless network, for facilitatingnoise and interference measurements on an air interface, said methodcomprising the steps of: obtaining, at a receiver, information relatingto a layout of a silent resource element (RE) grid for a first cell,wherein the silent RE grid indicates a plurality of resource elementsfor which no transmission is allocated on a data channel of the firstcell, measuring a signal that is received on the data channel within aresource element for which no data transmission is allocated for thefirst cell according to the silent RE grid; determining an amount ofinter-cell interference experienced on the data channel based on thesignal measurement; and reporting, to a transmitter, a channel qualityindication for the data channel based on the determined amount ofinter-cell interference.
 2. The method of claim 1, wherein obtaininginformation relating to the layout of the silent RE grid comprises:deriving a seed for a pseudo-random sequence generator; and using thepseudo-random sequence generator to generate the layout of the silent REgrid based on the seed.
 3. The method of claim 2, further comprisingsynchronizing the pseudo-random sequence generator based on a cellidentifier and a frame index.
 4. The method of claim 1, whereinobtaining information relating to the layout of the silent RE gridcomprises receiving signaling indicating the layout.
 5. The method ofclaim 1, wherein determining an amount of inter-cell interferenceoccurring on the data channel based on the signal measurement comprisesestimating one or more of a channel quality indicator, an inter-cellinterference power occurring on the data channel, a co-variance matrixfor inter-cell interference occurring on the data channel, a fullprobability distribution of inter-cell interference occurring on thedata channel.
 6. The method of claim 1, further comprising the step ofrandomizing the silent RE grid in consecutive, in frequency and time,resource blocks (RBs).
 7. The method of claim 1, wherein the silent REgrid comprises a first silent RE grid and the method further comprises:obtaining, at the receiver, information relating to a layout of a secondsilent RE grid for a control channel of the first cell, wherein thesecond silent RE grid indicates a plurality of resource elements forwhich no transmission is allocated on the control channel in the firstcell; measuring a second signal that is received on the control channelwithin a resource element for which no data transmission is allocatedfor the first cell according to the second silent RE grid; determiningan amount of inter-cell interference experienced on the control channelbased on the second signal measurement; and reporting, to thetransmitter, a second channel quality indication for the control channelbased on the determined amount of inter-cell interference on the controlchannel.
 8. The method as described in claim 1, further comprisingcoordinating the silent RE grid of the first cell with one or moresilent RE grids used in one or more neighboring cells, such that thesilent RE grid of the first cell is not identical to the one or moresilent RE grids used in one or more neighboring cells.
 9. The method asdescribed in claim 1, wherein the silent RE grid comprises, for eachsubframe, a plurality of resource windows, wherein each resource windowcomprises a group of contiguous, in frequency, resource blocks, andwherein each resource window includes a same number of silent REs. 10.The method of claim 9, further comprising semi-statically configuring,for the receiver, the number of resource blocks in each resource windowand the number of silent REs in each resource window.
 11. A receivingapparatus for use in a wireless network, the apparatus comprising: anetwork interface configured to obtain information relating to a layoutof a silent resource element (RE) grid for a first cell, wherein thesilent RE grid indicates a plurality of resource elements for which notransmission is allocated on a data channel of the first cell; and aprocessing unit configured to: measure a signal that is received on thedata channel within a resource element for which no data transmission isallocated for the first cell according to the silent RE grid; determinean amount of inter-cell interference experienced on the data channelbased on the signal measurement; and report, to a transmitter, a channelquality indication for the data channel based on the determined amountof inter-cell interference.
 12. The apparatus of claim 11, wherein theprocessing unit is configured to obtain information relating to thelayout of the silent RE grid by: deriving a seed for a pseudo-randomsequence generator; and using the pseudo-random sequence generator togenerate the layout of the silent RE grid based on the seed.
 13. Theapparatus of claim 12, wherein the processing unit is further configuredto synchronize the pseudo-random sequence generator based on a cellidentifier and a frame index.
 14. The apparatus of claim 11, wherein thenetwork interface is configured to obtain information relating to thelayout of the silent RE grid by receiving signaling indicating thelayout.
 15. The apparatus of claim 11, wherein the processing unit isconfigured to determine an amount of inter-cell interference experiencedby the receiver based on the signal measurement by estimating one ormore of a channel quality indicator, an inter-cell interference poweroccurring on the data channel, a co-variance matrix for inter-cellinterference occurring on the data channel, a full probabilitydistribution of inter-cell interference occurring on the data channel.16. The apparatus of claim 11, wherein the processing unit is furtherconfigured to randomize the silent RE grid in consecutive, in frequencyand time, resource blocks (RBs).
 17. The apparatus of claim 11, wherein:the silent RE grid comprises a first silent RE grid; the networkinterface is further configured to obtain information relating to alayout of a second silent RE grid for a control channel of the firstcell, wherein the second silent RE grid indicates a plurality ofresource elements for which no transmission is allocated on the controlchannel in the first cell; and the processing unit is further configuredto: measure a second signal that is received on the control channelwithin a resource element for which no data transmission is allocatedfor the first cell according to the second silent RE grid; determine anamount of inter-cell interference experienced on the control channelbased on the second signal measurement; and report, to the transmitter,a second channel quality indication for the control channel based on thedetermined amount of inter-cell interference on the control channel. 18.The apparatus of claim 11, wherein the processing unit is furtherconfigured to coordinate the silent RE grid of the first cell with oneor more silent RE grids used in one or more neighboring cells, such thatthe silent RE grid of the first cell is not identical to the one or moresilent RE grids used in one or more neighboring cells.
 19. The apparatusof claim 11, wherein the silent RE grid comprises, for each subframe, aplurality of resource windows, wherein each resource window comprises agroup of contiguous, in frequency, resource blocks, and wherein eachresource window includes a same number of silent REs.
 20. The apparatusof claim 19, wherein the processing unit is further configured tosemi-statically configure, for the receiver, the number of numberresource blocks in each resource window and the number of silent REs ineach resource window.